Tightness testing methods are known for closed containers in which a pressure differential relative to the environment is produced between the interior of the container and its surroundings, in a test chamber, by applying a suction source to the test chamber. Leaking containers are identified as follows: following application of a given pressure differential, a suction source required for the purpose is uncoupled from the test chamber and the change as a function of time in the pressure differential between the interior of the container and its environment is observed and/or recorded in basic fashion. This is accomplished for example by measuring the pressure values in the environment of the container at at least two points in time. Depending on the size of a leak, pressure equalization between the interior of the container and its environment proceeds faster or slower. Information on such a technique can be found for example in WO94105991 of the same applicant as for the present application.
The claimed procedure, especially according to the abovementioned WO94105991, makes it possible to detect extremely small leaks in containers. Problems arise when containers to be tested are filled at least partially with a flowable filling, especially with a low-pressure differntial between a liquid filling in the container and the environment, and the effect of suction, if a leak is present, causes liquid filling to escape into the environment through the leak, in other words at the outside wall of the container. Because of the sealing action of the escaping liquid filling, a leak in a container wall area exposed to a fluid can only be detected, if at all, by measuring pressure for relatively long periods of time, which is highly disadvantageous especially when testing containers arriving sequentially on a production line.
The goal of the present invention is therefore to propose a method of the species recited at the outset in which the disadvantages of the abovementioned pressure measurement test are eliminated. For this purpose, the said method according to the invention is characterized by the following: if, as in the case described above, liquid filling escapes to the exterior through a leak in the container wall because of the applied pressure differential, this is detected by electrical impedance measurement as proposed by the invention.
The liquid filling escaping in the immediate vicinity of the outside wall of the container produces a change in the electrical impedance between at least one pair of impedance measuring electrodes applied in this area, and an impedance measurement makes it possible to detect the change in this impedance produced by the filling.
While it is appropriate in certain cases to determine the escape of the filling by measuring the electrical alternating voltage impedance, especially when testing containers with electrically insulating walls, like those with plastic walls, and fillings that are electrical conductors, It is proposed to perform DC and preferably low-voltage DC resistance measurement as impedance measurement, using for example DC voltages below 50 V.
Although in cases when very specific locations on the container under test are to be tested for leaks, a single impedance-measurement location in the vicinity of the container section to be tested may suffice, It is also proposed to provide a plurality of impedance measurement sections connected in parallel and to arrange these along the container to be tested in order to detect leaks anywhere in the container.
This tightness testing method based on impedance measurement can now be combined according to the invention in a highly advantageous manner with the abovementioned pressure measurement test. Namely, when containers are to be tested which, as is usually the case, are only partially filled with flowable filling, so that basically there are air inclusions in the container in addition to the liquid filling, it is never certain where the air is located in the container and where the liquid is located. The fact that in addition to impedance measurement for differentiating between tightness and leakiness, the time curve of a pressure differential is recorded especially by tracking the pressure in an encapsulated container environment, regardless of whether air inclusions are present or where said inclusions are located at the moment in the container, whereby a combined determination of the leakage state of a container can be obtained: at those parts of the container wall currently exposed to the filling, the electrical impedance measurement is representative of leakage, while as far as the wall parts that are currently in contact with air inclusions are concerned, the pressure differential that is recorded is representative of leakage. Here again, the known highly precise technique from WO94/105991 is used to distinguish between tightness and leakiness on the basis of the pressure differential that develops, wherein, after establishing a predetermined vacuum between the interior of the container and the encapsulated environment and after disconnecting the system from a vacuum source, the pressure in the encapsulated environment is recorded at at least two points in time and the pressure differential is evaluated as an indication of tightness. To create an extremely sensitive measurement method, at the first point in time, with the recorded pressure value stored as a reference signal, a zero-deviation signal is also measured and at the second point in time the pressure differential is recorded relative to the zero-corrected value for the first point in time. This makes it possible to amplify the abovementioned differential or the evaluation signal corresponding to this differential in order to achieve high resolution.
It is highly advantageous in this connection to evaluate a signal indicating the existence of an impedance differential by using the same method as for evaluating any pressure differential that develops.
A test chamber according to the invention for tightness testing of closed containers with flowable filling, comprises a test chamber closable in a vacuum-tight manner, at least one impedance-measuring section being provided in the test chamber. The impedance-measuring section has at least one pair of spaced electrodes. In a disclosed embodiment, a plurality of distributed electrode pairs is provided in the test chamber. The pairs are, preferably, connected in parallel. In one form of the invention, the chamber inside wall is formed by a pattern of electrically conducting electrode sections and insulating sections separating the electrode sections from one another. A further feature of the invention involves providing at least one pressure sensor on the test chamber. There is also at least one cleaning gas connection terminating in the chamber.
A test system with at least one such test chamber is defined according to the invention with at least one electrode pair of the test chamber being effectively connected with an impedance measuring unit. In one embodiment, the impedance-measuring unit is a DC resistance measuring unit, preferably a low-voltage resistance measuring unit. The distinction between non-leaking and leaking is made using a threshold-value-sensitive unit on an evaluation unit. The at least one test chamber has a plurality of electrode pairs which are connected effectively in parallel with the input impedance-measuring unit. The impedance-measuring unit comprises a threshold valve-sensitive unit on the output side.
The test system further comprises a pressure sensor on the test chamber. The output of the pressure sensor is actively connected with an evaluation unit that preferably records the output signal at a first point in time and also at a second subsequent point in time, and feeds the recorded pressure sensor output signals to a differential unit whose output acts on the threshold value-sensitive unit. The two inputs of the differential unit receive the output signal of the sensor recorded at the first point in time and a zero-differential signal is formed and stored as the output signal of the differential unit. Both the pressure sensor and electrode pair are effectively connected with the same evaluation unit which, preferably switchably, generates a signal as a function of the impedance at the electrode pair and the sensor output signal. advantageous design of this test system is achieved by virtue of the fact that in the most preferred version, with both an impedance measurement and a pressure measurement, one and the same evaluation unit is used. For example, if a DC voltage is applied across a measuring resistance and the voltage across the measuring resistance is evaluated as the measurement signal for impedance measurement in the section between the preferably several electrode sections connected in parallel, the evaluation unit is supplied with a voltage, namely the voltage that depends on the fixed measuring resistance and the current that varies with the measurement section resistance. The evaluation unit itself is then a voltage-measuring device. By switching to a pressure-measuring sensor provided on the test chamber, this same evaluation unit can be used to measure pressure-dependent sensor output voltage.
A tester of the type according to the invention comprises a plurality of the testing systems. A central impedance-measuring unit is provided for the testing systems. The unit is switchable to individual testing systems. A central evaluation unit is effectively connectable in a switchable fashion with pressure sensors of the test chambers of the test systems and their electrode pair. It is especially advantageous that an evaluation unit is provided centrally for pressure measurement by several test systems provided with at least one test chamber, and an evaluation unit is provided for impedance measurement, each unit being switchable between the test systems or a single evaluation unit being provided that can be switched between the individual test systems and each of the individual test systems can be switched between pressure measurement and impedance measurement.
The method according to the invention, the test chamber, the test system, and the tester are preferably used for testing containers with electrically insulating walls, preferably glass or plastic walls, especially containers in the medical field such as plastic ampoules. At the same time, the tightness test is advantageously performed on several containers forming a set of containers, with leaks in one of these containers or one of these ampoules resulting in nonselective rejection of the entire set.